1. Field of the Invention
The present invention relates generally to the calibration of multiple transceiver systems and more particularly to calibrating aspects of two or more transceivers without requiring dedicated circuits.
2. Description of the Related Art
Most radio transceivers require some form of calibration to compensate for variations in characteristics between the devices/systems of which they are a part, variations that occur over time, and variations due to their operating conditions/environment. Such calibration is necessary to maintain compliance with relevant standards, specifications and regulatory requirements, as well as necessary to maximise their performance. It is desirable for as much of this calibration as possible to be performed automatically by the device itself, without requiring any special external equipment or user interaction. This is especially true for consumer products such as mobile phones and routers where performing calibrations as part of the production process increases cost, and the end user will expect the device to “just work”.
Furthermore, more and more electronic products incorporate multiple independent (or mostly independent) radio transceivers. For example, most IEEE 802.11n or IEEE 802.11ac systems/devices (other than some small form factor handheld devices like mobile phones) utilize multiple antennas to improve throughput and range using Multiple Input Multiple Output (MIMO) techniques, where each antenna is connected to its own dedicated transceiver.
Two approaches are commonly used to perform calibrations in multi-transceiver systems/devices:                1) Inject and/or measure signals at appropriate points within the signal path of the receiver and/or transmitter that is being calibrated. This is typically done with dedicated Analog-to-Digital Convertors (ADCs), Digital-to-Analog Convertors (DACs), or oscillators that are used only for calibration purposes.        2) Looping back (“loopback”) the signal from the transmit path of the transmitter into the receive path of the receiver. This is often done at multiple points along the transmit and receive paths, such as from the DACs to the ADCs, and from the output of the transmit mixer to the input of the receive mixer.        
Both of these approaches require dedicated hardware, which increases the design effort and the cost of the product (e.g., increased silicon area for the case of an Integrated Circuit (IC) implementation). More importantly, these approaches typically do not support full calibration of the final stages of the transmitter (such as, notably, the Power Amplifier (PA)), the first stages of the receiver (such as, e.g., the Low Noise Amplifier (LNA)), or any off-chip external components.
For example, a commonly deployed loopback approach is to loop the signal from near the end of the transmit path back into either the receive path of the same transceiver or dedicated calibration circuits. Loopback into the receiver is usually only implemented for the final stage within a particular Integrated Circuit (IC), so for a device with an external Power Amplifier (PA) it would normally exclude using the loopback for calibrating the gain steps or distortion characteristics of the PA itself. Loopback via dedicated calibration circuits usually has significantly reduced bandwidth (or a high implementation cost for full bandwidth), e.g. the signal from an external PA would normally be measured by a low frequency peak detector or power detector, which again is unsuitable for calibrating the PA's distortion characteristics. Moreover, these dedicated calibration circuits also suffer from variable and non-ideal characteristics. Taking the example of PA peak detectors, they typically have a response that is non-linear with output power, and that varies between devices, with temperature and with RF frequency.
Thus, there is a need for an approach to calibrating radio transceivers, including their final/first stages, in multiple transceiver devices/systems without requiring additional hardware.